JP2006-111766A (hereinafter referred to as Reference 1) discloses a desulfurization unit for liquefied petroleum gas (LPG). The desulfurization unit includes a first desulfurizer operating in a normal temperature and a second desulfurizer operating in a high temperature (greater than 100° C.). The first desulfurizer is arranged at an upstream side in view of a flow direction of a source gas. The first desulfurizer is provided for a sulfur compound (tertiary-butylmercaptan (TBM) or dimethyl sulfide (DMS), for example) that serves as an odorant used in the source gas and that is relatively easy to be desulfurized. The second desulfurizer is arranged at a downstream side relative to the first desulfurizer. The second desulfurizer is provided for a sulfur compound specifically included in LPG (carbonyl sulfide (CO), for example). Metal oxide such as nickel is used in the second desulfurizer.
JP2006-265480A (hereinafter referred to as Reference 2) discloses a desulfurization unit basically including the same configuration as that of Reference 1. Specifically, according to Reference 2, the desulfurization unit includes a first desulfurizer operating in a normal temperature and a second desulfurizer operating in a high temperature. The second desulfurizer operates at 50° C. or greater. A desulfurizing agent accommodated in the second desulfurizer is provided for LPG. JP05-114414A (hereinafter referred to as Reference 3) includes a first desulfurizer operating in a normal temperature and a second desulfurizer operating in a high temperature, the second desulfurizer serving as a hydrogenation desulfurizer. That is, according to Reference 3, two desulfurization methods are combined so that the first desulfurizer covers an insufficiency of a desulfurization effect of the hydrogenation desulfurizer when a fuel cell system is started.
According to Reference 1, the second desulfurizer operating in the high temperature adsorbs the sulfur compound specifically included in LPG by metal oxide, for example, thereby removing the sulfur compound from LPG. The second desulfurizer is not intended to inhibit the desulfurizing agent from being damaged by water vapor included in the source gas. According to Reference 3, the second desulfurizer operating in the high temperature is intended to perform a hydrogenation desulfurization so that hydrogen is necessary for the desulfurization.
For example, the source gas corresponding to a city gas supplied by a gas company may include water vapor. The desulfurizing agent used in a normal temperature environment (which will be hereinafter referred to as a normal-temperature desulfurizing agent) is generally formed by a porous material such as zeolite and activated carbon. The normal-temperature desulfurizing agent adsorbs the sulfur compound in the source gas for the desulfurization. The normal-temperature desulfurizing agent formed by the porous material as a base material is inexpensive. However, in a case where a source gas having a high dew point and including a relatively large volume of water vapor is desulfurized by the normal-temperature desulfurizing agent, for example, the normal-temperature desulfurizing agent may preferentially adsorb the water vapor to the sulfur compound in the source gas. As a result, the adsorption capacity of the normal-temperature desulfurizing agent relative to the sulfur compound in the source gas may drastically decrease, therefore deteriorating the desulfurization effect of the normal-temperature desulfurizing agent (see FIG. 12).
A source gas having a low dew point and including a relatively small volume of water vapor is generally supplied to the industrial world. However, when a gas fitting work or a gas-pipe laying work is performed, for example, the water vapor included in the source gas increases; therefore, the dew point of the source gas may increase. At this time, the adsorption capacity of the desulfurizing agent relative to the sulfur compound in the source gas is reduced because of the water vapor included in the source gas having the high dew point. As a result, the desulfurization effect of the desulfurizing agent may decrease. In order to resolve such issue, a use amount of desulfurizing agent my excessively increase, which results in a cost increase and an enlargement of a fuel cell system. Further, a position and/or an improvement of a lifetime of a flowmeter, for example, is not considered or disclosed according to References 1 and 2.
A need thus exists for a fuel cell system which is not susceptible to the drawback mentioned above.